Having been to Cozumel on many occasions by cruise ship as well as staying there for a couple of weeks on vacation, and being a very active scuba diver for the last 17 years, I can tell you that snorkeling in Cozumel is good, but not great. Fish like to stay around the coral reef - it provides food and shelter. Problem is, the really good reef doesn't start until around 45' deep. There is some reef closer to the surface, but compared to other places, it's disappointing.
On your itinerary, if you want to do some nice snorkeling, then I would suggest looking at Grand Cayman instead. The reef is closer to the surface and you can see more fish. Plus, you can do it right near the tender pier and if you know where to go and have your own equipment, it's free!
__________________ 46 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!